Silicon nitride filter and method for its production

ABSTRACT

The present invention provides a silicon nitride filter which is a filter excellent in heat resistance, thermal shock resistance, corrosion resistance, acid resistance and mechanical strength and suitable for dust arresting or dust removing, and which is particularly useful as a filer for particulates, and a method for its production.  
     The present invention provides a method for producing a silicon nitride filter, characterized by heat-treating in nitrogen a green body comprising from 40 to 90% of metal silicon particles having an average particle diameter of from 1 to 200 μm and from 10 to 60% of a pore-forming agent, provided that the total amount of the metal silicon particles and the pore-forming agent is at least 90%, to form a porous product made substantially of silicon nitride.  
     The present invention provides a silicon nitride filter characterized in that the porosity is from 40 to 70%, and the cumulative pore volume of pores with diameters of at most 1 μm is from 1 to 15 vol % of the total pore volume.

TECHNICAL FIELD

[0001] The present invention relates to a silicon nitride filtersuitable for removing dust, etc. contained in a high temperature exhaustgas, and a method for its production.

BACKGROUND ART

[0002] Heretofore, a cordierite type ceramic filter or silicon carbidetype ceramic filter has been proposed as a filter to remove dust, etc.contained in a high temperature exhaust gas. However, the cordieritetype ceramic filter is not necessarily adequate from the viewpoint ofheat resistance and corrosion resistance although it is excellent inthermal shock resistance, and the silicon carbide type ceramic filter isnot necessarily adequate with respect to the thermal shock resistance,although it is excellent in heat resistance and corrosion resistance.

[0003] Particularly when the ceramic filter is one intended forarresting diesel particulates (hereinafter referred to simply asparticulates) discharged from a diesel engine (hereinafter referred tosimply as an engine), it has been likely with the above-mentionedcordierite type filter or the silicon carbide type filter that theparticulates arrested by the filter will locally burn to cause a meltingloss, thus presenting a fatal damage to the ceramic filter. Further, theparticulates contain a sulfur content and a phosphor content, wherebyacid resistance is required, but the cordierite type filter used to benot necessarily adequate with respect to the acid resistance.

[0004] On the other hand, silicon nitride has excellent characteristicswith respect to heat resistance, thermal shock resistance, corrosionresistance, acid resistance, mechanical strength, etc., and is expectedto be useful as a filter for dust arresting or dust removing in a hightemperature or corrosive environment. Especially, silicon nitride isexcellent in heat resistance, thermal shock resistance, acid resistanceand mechanical strength, and is thus considered to be a materialsuitable for a filter for particulates.

[0005] As a method for producing such a silicon nitride filter, severalhave been proposed.

[0006] For example, JP-A-6-256069 proposes a method of firing a greenbody comprising silicon nitride particles, clay and an oxide. Further,JP-A-7-187845, JP-A-8-59364 and JP-A-6-24859 propose methods of using asstarting materials a mixture comprising silicon nitride particles and anorganic silicon compound, a mixture comprising silicon nitride particlesand a polysilazane and a mixture comprising silicon nitride particlesand a synthetic resin foam, respectively. However, such methods of usingsilicon nitride particles as starting materials have had a problem thatas compared with a method of using metal silicon particles as thestarting material and converting it to silicon nitride by directnitriding, pores with pore diameters of at most 1 μm are little, wherebythe Young's modulus is high, the thermal shock resistance tends to bepoor, the production cost tends to be problematic since the siliconnitride particles are relatively expensive.

[0007] On the other hand, as a method of employing metal siliconparticles, JP-A-1-188479 proposes a production method to obtain a porousproduct having a nitriding ratio of the metal silicon particles of atmost 50%, by using as a starting material a mixture comprising metalsilicon particles and silicon nitride particles. However, in thismethod, the nitriding ratio of the metal silicon particles is at most50%, whereby there will be a substantial amount of silicon metalremaining in the silicon nitride sintered body in the form of metalsilicon without being nitrided, whereby there will be a problem suchthat excellent heat resistance or corrosion resistance of siliconnitride will be impaired.

[0008] Further, by the method of using metal silicon particles,sintering of the formed silicon nitride particles is usually notsufficient, whereby the mechanical strength of the porous body therebyobtained, tends to be inadequate.

DISCLOSURE OF THE INVENTION

[0009] The present invention provides a method for producing a siliconnitride filter, which comprises heat-treating in nitrogen a green bodycomprising from 40 to 90 mass % (hereinafter referred to simply as %) ofmetal silicon particles having an average particle diameter of from 1 to200 μm and from 10 to 60% of a pore-forming agent, provided that thetotal amount of the metal silicon particles and the pore-forming agentis at least 90%, to form a porous product made substantially of siliconnitride.

[0010] Another invention of the present invention provides a siliconnitride filter characterized in that the porosity is from 40 to 70%, andthe cumulative pore volume of pores with diameters of at most 1 μm isfrom 1 to 15 vol % of the total pore volume.

BEST MODE FOR CARRYING OUT THE INVENTION

[0011] In the method for producing a silicon nitride filter of thepresent invention, a green body is used which comprises from 10 to 60%of a pore-forming agent and from 40 to 90% of metal silicon particleshaving an average particle diameter of from 1 to 200 μm, provided thatthe total amount of the metal silicon particles and the pore-formingagent is at least 90%.

[0012] If the pore-forming agent is less than 10%, the proportion ofpores to perform a filter function tends to be inadequate, and if thepore-forming agent exceeds 60%, no adequate strength tends to beobtained, although the porosity of the filter becomes large. Further, ifthe average particle diameter of the metal silicon particles is lessthan 1 μm, the amount of moisture or oxygen adsorbed from outside airduring e.g. preparation of the green body tends to increase, and whenheat treated, the metal silicon particles tend to be oxidized beforebeing nitrided, whereby the amount of silicon dioxide formed, tends tobe too large. Further, if the average particle diameter of the metalsilicon particles exceeds 200 μm, metal silicon particles not nitridedtend to remain in the interior of the sintered body even after the heattreatment, whereby the properties as the silicon nitride filter tend todeteriorate. If the metal silicon particles are less than 40%, themerits of using metal silicon particles, i.e. the merit of using thedirect nitriding reaction of metal silicon, will not be utilized. On theother hand, if the content of the metal silicon particles exceeds 90%,the content of the pore-forming agent tends to be small, whereby theporosity cannot be made large. The purity of the metal silicon particlesmay suitably be selected depending upon the purpose and application.

[0013] In this specification, the pore-forming agent is not particularlylimited so long as it forms pores. The pore-forming agent may, forexample, be one which flies upon e.g. decomposition during heattreatment, to form pores (hereinafter referred to as a flying-typepore-forming agent) or oxide ceramic hollow particles.

[0014] The heat-treating conditions are preferably two-step heattreatment in a nitrogen atmosphere, which is divided into a first stepsuitable for nitriding metal silicon particles and a second stepsuitable for sintering silicon nitride particles as formed nitride.

[0015] As the heat-treating conditions of the first step, it ispreferred to maintain in a nitrogen atmosphere from 1000 to 1400° C. forfrom 4 to 24 hours. If the temperature is lower than 1000° C., nitridingof metal silicon particles tends to hardly take place. On the otherhand, if the temperature exceeds 1400° C., the metal silicon particleswill melt in the vicinity of the melting point (1410° C.) of metalsilicon, whereby the shape cannot be maintained, such being undesirable.If the temperature maintaining time is less than 4 hours, nitriding ofmetal silicon particles tends to be inadequate, such being undesirable.Further, if the temperature maintaining time exceeds 24 hours, thenitriding reaction will no more substantially proceed, and the operationcost increases, such being undesirable.

[0016] As heat-treating conditions of the second step, it is preferredto maintain in a nitrogen atmosphere at from 1450 to 1800° C. for from 1to 12 hours. If the temperature is lower than 1450° C., sintering of thesilicon nitride particles tends to hardly proceed, such beingundesirable, and if it exceeds 1800° C., the silicon nitride particlestend to be decomposed, such being undesirable. If the temperaturemaintaining time is less than 1 hour, bonding of the particles to oneanother will not adequately proceed, such being undesirable. On theother hand, if it exceeds 12 hours, silicon nitride tends to bedecomposed, such being undesirable. The heat treatment of the first stepand the heat treatment of the second step may be carried outcontinuously without lowering the temperature or the temperature may beonce lowered at an intermediate point.

[0017] The temperature raising rate at the time of the heat treatmentmay suitably be selected depending upon the size, shape, etc. of thegreen body. However, it is preferably from 50 to 600° C./hr from theviewpoint of the nitriding ratio and the pore diameter. Even in aprocess of temperature raising, if the temperature is within atemperature range prescribed for the first step or the second step, thetime passed will be included in the maintaining time for the first stepor the second step.

[0018] Here, the nitrogen atmosphere is an atmosphere containingsubstantially solely nitrogen without containing oxygen, but it maycontain other inert gas. The nitrogen partial pressure is preferably atleast 50 kPa.

[0019] In the method for producing a silicon nitride filter of thepresent invention, the pore-forming agent is preferably oxide ceramichollow particles. The method for producing a silicon nitride filter ofthe present invention wherein the pore-forming agent is oxide ceramics,will hereinafter be referred to as method 1.

[0020] In method 1, it is preferred to use a green body which comprisesfrom 15 to 50% of oxide ceramic hollow particles and from 40 to 85% ofmetal silicon particles having an average particle diameter of from 5 to200 μm, provided that the total amount of the oxide ceramic hollowparticles and the metal silicon particles, is at least 90%.

[0021] Oxide ceramic hollow particles (hereinafter referred to as hollowparticles) may be any particles so long as they are capable of formingpores during the heat treatment and they serve as a sintering aid to thesilicon nitride particles formed in the heat-treating step.

[0022] It is preferred that the hollow particles contain as the maincomponent an oxide of at least one metal selected from the groupconsisting of Al, Si, Ca, Sr, Ba and Mg, since the effect as a sinteringaid is thereby high.

[0023] The hollow particles may have outer skin portion being dense orporous so long as they are hollow. Further, the hollow particles arepreferably spherical particles, as they are readily available, butparticles other than spherical particles may be employed so long as theyare hollow.

[0024] The average particle diameter of the hollow particles ispreferably from 30 to 200 μm, whereby the porosity of the filter to beobtained, will be large, and the strength will be secured. If theaverage particle diameter of the hollow particles is less than 30 μm,the contribution to formation of pores will decrease. On the other hand,if the average particle diameter exceeds 200 μm, the strength of thefilter to be obtained tends to be inadequate, such being undesirable.The content of the hollow particles is preferably from 15 to 50% of thegreen body.

[0025] With the metal silicon particles to be used in method 1, theaverage particle diameter is preferably from 5 to 200 μm, morepreferably from 30 to 150 μm.

[0026] In method 1, the total amount of the hollow particles and themetal silicon particles will be at least 90% in the green body.

[0027] In method 1, a common mixing means such as a ball mill or a mixermay be employed for mixing the hollow particles and the metal siliconparticles, and a common ceramic-forming method such as press molding,extrusion molding or slip casting, may suitably be employed as a methodfor preparing a green body comprising hollow particles and metal siliconparticles. Further, at the time of molding, an organic binder may beincorporated. As such an organic binder, an organic substance such aspolyvinyl alcohol or its modified product, starch or its modifiedproduct, carboxymethylcellulose, hydroxylmethylcellulose, polyvinylpyrrolidone, an acrylic resin or an acrylic copolymer, a vinyl acetateresin or a vinyl acetate copolymer, may, for example, be employed. Theamount of such an organic binder is preferably from 1 to 10 parts bymass (hereinafter referred to simply as parts) per 100 parts of thegreen body.

[0028] As conditions for heat treating the above green body, it ispreferred that the heat-treating conditions of the first step are tomaintain the green body in a nitrogen atmosphere at from 1200 to 1400°C. for from 4 to 12 hours, and the heat-treating conditions of thesecond step are to maintain it in a nitrogen atmosphere at from 1500 to1800° C. for 1 to 12 hours.

[0029] The porosity of the silicon nitride filter obtained by method 1,is preferably from 30 to 80%. The porosity is measured by an Archimedeanmethod. If the porosity is less than 30%, the pressure loss tends to belarge, such being undesirable as a filter. On the other hand, if theporosity exceeds 80%, the strength tends to be low, such beingundesirable as a filter.

[0030] The average pore diameter as measured by a mercury immersionmethod of the silicon nitride filter obtained by method 1, is preferablyfrom 5 to 40 μm. If the average pore diameter is less than 5 μm, thepressure loss during the use of the filter tends to be large, such beingundesirable. If the average pore diameter exceeds 40 μm, arresting andremoving fine exhaust particles such as diesel particulates tend to bedifficult, such being undesirable.

[0031] In the method for producing a silicon nitride filter of thepresent invention, the pore-forming agent is preferably a flying-typepore-forming agent. The method for producing a silicon nitride filter ofthe present invention wherein the pore-forming agent is a flying-typepore-forming agent, will be hereinafter referred to as method 2.

[0032] In method 2, it is preferred to use a green body which comprisesfrom 10 to 50% of the flying-type pore-forming agent and from 40 to 90%of metal silicon particles having an average particle diameter of from 1to 30 μm, provided that the total amount of the flying-type pore-formingagent and the metal silicon particles is at least 90%.

[0033] As the flying-type pore-forming agent, either an organicsubstance or an inorganic substance may be suitably employed so long asit is capable of flying upon e.g. decomposition during the heattreatment, to form pores. It is preferred that the flying-typepore-forming agent is organic polymer particles, particularly heatdecomposable polymer particles, since they will decompose and fly in theheat treatment process without leaving any residue in the sintered body,whereby properties of the obtainable silicon nitride porous product willnot be impaired.

[0034] As such an organic polymer, polyvinyl alcohol, an acrylic resin,a vinyl acetate resin or cellulose, may, for example, be mentioned. Iforganic polymer particles added as a flying-type pore-forming agentduring the temperature raising will not sufficiently be heat-decomposedin the temperature raising step in the heat treatment and will remain ina substantial amount as carbon, silicon carbide will be formed in thesubsequent heat treating process, whereby pores are likely to beclogged, such being undesirable. From this viewpoint, it is preferred touse acrylic resin particles as a flying-type pore-forming agent, wherebyit is readily heat decomposable, and the amount remaining as carbon willbe little.

[0035] The content of the flying-type pore-forming agent is preferablyfrom 10 to 50% in the green body, more preferably from 15 to 40%,whereby both the strength and porosity of the filter can be made high.

[0036] Further, it is particularly preferred that the flying-typepore-forming agent is spherical, whereby pores to be formed will also bespherical, and deterioration of the strength can be suppressed even whenthe porosity is made high. Further, when the flying-type pore-formingagent is spherical, the average particle diameter is preferably from 20to 100 μm. If the average particle diameter of the flying-typepore-forming agent is less than 20 μm, the average pore diameter of thesilicon nitride filter obtained after the heat treatment will be nothigher than 5 μm, such being undesirable. On the other hand, if itexceeds 100 μm, the average pore diameter of the silicon nitride filterobtained after the heat treatment will be at least 20 μm, such beingundesirable as a filter for e.g. dusts.

[0037] The metal silicon particles to be used in method 2 preferablyhave an average particle diameter of from 1 to 30 μm. The content of themetal silicon particles is preferably from 40 to 90%, more preferablyfrom 50 to 80%, in the green body. In method 2, the total amount of theflying-type pore-forming agent and the metal silicon particles is atleast 90% in the green body. If the total amount of the flying-typepore-forming agent and the metal silicon particles, is less than 90% inthe green body, it is impossible to obtain a filter having the desiredproperties.

[0038] In method 2, as a method for forming a green body comprising theflying-type pore-forming agent and the metal silicon particles, a usualceramic molding method as mentioned above, may suitably be employed.Further, at the time of the molding, an organic binder may be addedseparately from the flying-type pore-forming agent. As such an organicbinder, the above-mentioned binder may preferably be employed. Theamount of such an organic binder is preferably from 1 to 10 parts per100 parts of the green body. Further, the flying-type pore-forming agentmay serve as a binder for the green body.

[0039] In method 2, the heat-treating conditions of the first step arepreferably such that the green body is maintained in a nitrogenatmosphere at from 1100 to 1400° C. for from 5 to 24 hours. Further, theheat-treating conditions of the second step are preferably such that thegreen body is maintained in a nitrogen atmosphere at from 1450 to 1800°C. for from 2 to 5 hours.

[0040] The porosity of the silicon nitride filter obtained by method 2,is preferably from 30 to 80%. The porosity is measured by an Archimedeanmethod. If the porosity is less than 30%, the pressure loss becomeslarge, such being undesirable as a filter. Further, if the porosityexceeds 80%, the strength tends to be low, such being undesirable as afilter.

[0041] The average pore diameter as measured by a mercury immersionmethod of the silicon nitride filter obtained by method 2, is preferablyfrom 5 to 20 μm. If the average pore diameter is less than 5 μm, thepressure loss during use of the filter tends to be large, such beingundesirable. If the average pore diameter exceeds 20 μm, it tends to bedifficult to arrest and remove fine exhaust particles such asparticulates, such being undesirable.

[0042] The ratio (hereinafter referred to as the nitriding ratio) ofsilicon contained as silicon nitride to total silicon of metal siliconof the silicon nitride filter obtained by method 2, is preferably atleast 90%. If the nitriding ratio is less than 90%, the properties suchas heat resistance and corrosion resistance of the silicon nitridefilter will be low due to the remaining metal silicon particles, suchbeing undesirable.

[0043] In this specification, the nitriding ratio of silicon nitride iscalculated from the change in mass. Namely, the reaction for formationof silicon nitride is such that as shown by the formula 1, 3 mols ofmetal silicon will react with 2 mols of nitrogen to form 1 mol ofsilicon nitride.

3Si+2N₂→Si₃N₄  Formula 1

[0044] From the formula 1, if metal silicon is all converted to siliconnitride, the mass will be 1.67 times((3×Si+4×N)/(3×Si)=(3×28+4×14)/(3×28)=1.67). If the change in mass isα-times, the nitriding ratio is calculated from(α−1)/(1.67-1)=(α-1)/0.67. For example, if it is 1.37 times, thenitriding ratio will be 55% (0.37/0.67×100=55%).

[0045] The silicon nitride filter of the present invention (hereinafterreferred to as the present filter) is characterized in that the porosityis from 40 to 70%, and the cumulative pore volume of pores withdiameters of at most 1 μm is from 1 to 15 vol % of the total porevolume. The present filter preferably has a Young's modulus of from 20to 100 GPa and a thermal expansion coefficient of at most 4×10⁻⁶/° C.The thermal expansion coefficient is a value within a temperature rangeof from room temperature to 1000° C.

[0046] The porosity of the present filter is from 40 to 70%. If theporosity is less than 40%, the pore volume will be too small, and thepressure loss will increase. On the other hand, if it exceeds 70%, themechanical strength as a filter tends to be inadequate.

[0047] The ratio of the cumulative pore volume of pores with diametersof at most 1 μm in the total pore volume (hereinafter referred to simplyas a 1 μm pore volume ratio) is from 1 to 15 vol %. If the 1 μm porevolume ratio is less than 1 vol %, the Young's modulus will be high, andthe thermal shock resistance will deteriorate. Further, if the 1 μm porevolume ratio exceeds 15 vol %, the pressure loss of the filter tends toincrease, or the mechanical strength tends to be low. Preferably, the 1μm pore volume ratio is from 5 to 10 vol %.

[0048] The Young's modulus of the present filter is preferably from 20to 100 GPa. If the Young's modulus is less than 20 GPa, the mechanicalstrength of the filter material tends to be too low. On the other hand,if it exceeds 100 GPa, the thermal stress generated by thermal shocktends to be large, whereby thermal shock resistance tends todeteriorate, such being undesirable.

[0049] In this specification, the pore volume is measured by a mercuryimmersion method, and the Young's modulus is calculated from Young'smodulus E (Pa)=σ/ε, by measuring the stress σ (Pa) and the strain ε, bythe tensile strength measurements. The method for measuring the strainmay, for example, be a method of using a strain gage.

[0050] For the measurement of the Young's modulus, the sample size is1×1×6 cm, and the longitudinal direction is the tensile direction. Thetensile load is applied at 0.5 mm/min. In a case where the sample is ahoneycomb, it is cut out so that the above-mentioned longitudinaldirection will be the extrusion direction during the molding i.e. willbe in parallel with the through holes, and at both ends, holes werefirmly sealed with e.g. an acrylic resin adhesive or an epoxy type resinadhesive for from 5 to 10 mm from the end surfaces. The strain ismeasured by attaching a strain gage to the sample.

[0051] The above-mentioned method 1 or method 2 is preferably employedas a method for producing the present filter.

EXAMPLES

[0052] Further, pores were measured by a mercury porosimeter(AUTOSCAN-33, trade name, manufactured by Yuasa Ionics K.K.).

Example 1 (Present Invention)

[0053] To 70 parts of metal silicon particles having an average particlediameter of 50 μm, 30 parts of alumina type hollow particles having anaverage particle diameter of 50 μm were added, and ethanol was furtheradded as a disperse medium, followed by wet mixing for 30 minutes by aball milling method and finally by drying. The obtained mixed powder wasfilled into a 40 mm×60 mm press mold, followed by uniaxial press moldingunder a pressing pressure of 20 MPa. After the molding, the green bodywas heated from room temperature to 1300° C. at a rate of 400° C./hr ina nitrogen atmosphere (nitrogen pressure=0.1 MPa) in anatmosphere-controlled electric furnace (hereinafter referred to simplyas an electric furnace) and maintained at 1300° C. for 8 hours, and thenit was heated to 1700° C. at a rate of 60° C./hr and maintained at 1700°C. for 5 hours for heat treatment.

[0054] The obtained sintered body had a porosity of 65% and an averagepore diameter of 20 μm. With respect to this porous product,identification of the crystal phases was carried out by X-rays, wherebyonly silicon nitride was observed. With respect to this porous product,the thermal expansion coefficient was measured, and it was a low thermalexpansion of 3.0×10⁻⁶/° C. within a range of from room temperature to1000° C. Further, from the sintered body, a bending test specimen of asize of 4 mm×3 mm×40 mm was cut out, and the three point bendingstrength with a span of 30 mm was measured at room temperature. The loadapplying rate was 0.5 mm/min. As a result, the bending strength was 50MPa.

Example 2 (Comparative Example)

[0055] The operation was the same as in Example 1 except that in Example1, the amount of the alumina type hollow particles added, was changedfrom 30 parts to 110 parts. The obtained sintered body had a porosity of88% and an average pore diameter of 35 μm. With respect to this porousproduct, the X-ray diffraction, measurement of the thermal expansioncoefficient and measurement of the three point bending strength werecarried out in the same manner as in Example 1. As a result, in theidentification of crystal phases, in addition to silicon nitride, a peakof alumina was observed. Further, the thermal expansion coefficient wasa low thermal expansion of 5.5×10⁻⁶/° C. within a range of from roomtemperature to 1000° C. The three point bending strength at roomtemperature was 5 MPa.

Example 3 (Comparative Example)

[0056] The operation was the same as in Example 1 except that in Example1, no alumina type hollow particles were added. The obtained sinteredbody had a porosity of 20% and an average pore diameter of 1.5 μm. Withrespect to this porous product, the X-ray diffraction, measurement ofthe thermal expansion coefficient and measurement of the three pointbending strength were carried out in the same manner as in Example 1. Asa result, in the identification of crystal phases, no peak other thansilicon nitride was observed. Further, the thermal expansion coefficientwas a low thermal expansion of 3.0×10⁻⁶/° C. within a range of from roomtemperature to 1000° C. The three point bending strength at roomtemperature was 250 MPa.

Example 4 (The Present Invention)

[0057] The operation was the same as in Example 1 except that in Example1, the average particle diameter of the metal silicon particles waschanged from 50 μm to 1 μm, and the alumina type hollow particles werechanged to spinel type hollow particles. The obtained sintered body hada porosity of 45% and an average pore diameter of 4 μm. With respect tothis porous product, the X-ray diffraction, measurement of the thermalexpansion coefficient and measurement of the three point bendingstrength were carried out in the same manner as in Example 1. As aresult, in the identification of crystal phases, in addition to siliconnitride, a peak of spinel was slightly observed. Further, the thermalexpansion coefficient was a low thermal expansion of 4.0×10⁻⁶/° C.within a range of from room temperature to 1000° C. The three pointbending strength at room temperature was 60 MPa. Further, in theobtained sintered body, a remarkable deformation was observed. This isconsidered to be attributable to the fact that sintering proceededremarkably due to the formed liquid phase.

Example 5 (Comparative Example)

[0058] The operation was the same as in Example 1 except that in Example1, the average particle diameter of the metal silicon particles waschanged from 50 μm to 400 μm. With respect to this porous product, theX-ray diffraction, measurement of the thermal expansion coefficient andmeasurement of the three point bending strength were carried out in thesame manner as in Example 1. As a result, in the identification ofcrystal phases, in addition to silicon nitride, peaks of silicon andalumina were observed. Further, the thermal expansion coefficient was alow thermal expansion of 4.8×10⁻⁶/° C. within a range of from roomtemperature to 1000° C. The three point bending strength at roomtemperature was 25 MPa.

Example 6 (The Present Invention)

[0059] To 75 parts of metal silicon particles having an average particlediameter of 50 μm, 25 parts of calcium sulfate powder having an averageparticle diameter of 100 μm granulated by a spray drying method wasadded as hollow particles, followed by dry mixing for 30 minutes by amixer. The obtained mixed powder was press-molded in the same manner asin Example 1. After the molding, it was heated in a nitrogen atmosphere(nitrogen pressure=0.1 MPa) in an electric furnace at a rate of 400°C./hr from room temperature to 500° C., at a rate of 60° C./hr from 500°C. to 1500° C. and at a rate of 300° C./hr from 1500 to 1600° C. andmaintained at 1600° C. for 10 hours to carry out nitriding treatment.

[0060] The obtained sintered body had a porosity of 55% and an averagepore diameter of 30 μm. With respect to this porous product,identification of the crystal phases was carried out by X-rays, wherebyonly silicon nitride was observed. With respect to this porous product,the thermal expansion coefficient was measured and found to be a lowthermal expansion of 3.1×10⁻⁶/° C. within a range of from roomtemperature to 1000° C., and the three point bending strength was 40MPa.

Example 7 (The Present Invention)

[0061] To 80 parts of metal silicon particles having an average particlediameter of 25 μm, 20 parts of silica-type glass hollow particles havingan average particle diameter of 45 μm, were added, and ethanol wasfurther added as a disperse medium, followed by wet mixing for 30minutes by a ball milling method, and finally by drying. The obtainedmixed powder was press-molded in the same manner as in Example 1. Afterthe molding, it was heated in a nitrogen atmosphere (nitrogenpressure=0.1 MPa) in an electric furnace at a rate of 400° C./hr fromroom temperature to 1100° C., maintained at 1100° C. for 10 hours, thenheated to 1700° C. at a rate of 60° C./hr and maintained at 1700° C. for5 hours to carry out heat treatment.

[0062] The obtained sintered body had a porosity of 50% and an averagepore diameter of 15 μm. With respect to this porous product, the thermalexpansion coefficient was measured and found to be a low thermalexpansion of 2.9×10⁻⁶/° C. within a range of from room temperature to1000° C., and the three point bending strength was 55 MPa.

Example 8 (The Present Invention)

[0063] To 75 parts of metal silicon particles having an average particlediameter of 20 μm, 25 parts of silica-alumina type hollow particleshaving an average particle diameter of 50 μm, were added, followed bydry mixing by a mixer. To 100 parts of this mixed powder, 10 parts ofmethylcellulose and 10 parts of deionized water were added andthoroughly kneaded by a kneader to obtain an extrusion molding mixture,followed by extrusion molding. The obtained extrusion green body wasdried by a warm air drier and then heated in a nitrogen atmosphere(nitrogen pressure=0.1 MPa) in an electric furnace at a rate of 400°C./hr from room temperature to 800° C., maintained at 800° C. for 2hours, then heated to 1700° C. at a rate of 60° C./hr and maintained at1700° C. for 5 hours to carry out heat treatment.

[0064] The obtained sintered body had a porosity of 60% and an averagepore diameter of 18 μm. With respect to this porous product,identification of crystal phases was carried out by X-rays, whereby onlysilicon nitride was observed. Further, with respect to this porousproduct, the thermal expansion coefficient was measured and found to bea low thermal expansion of 2.9×10⁻⁶/° C. within a range of from roomtemperature to 1000° C. The three point bending strength was 60 MPa.

Example 9 (The Present Invention)

[0065] To 70 parts of metal silicon powder having an average particlediameter of 40 μm, 30 parts of an acryl type organic spherical particleshaving an average particle diameter of 20 μm were added, and ethanol wasfurther added as a disperse medium, followed by wet mixing for 2 hoursby a ball milling and finally by drying. The obtained mixed powder waspress-molded in the same manner as in Example 1. After the molding, itwas heated in a nitrogen atmosphere (nitrogen pressure 0.1 MPa) in anelectric furnace at a rate of 60° C./hr from room temperature to 500°C., then at a rate of 400° C./hr from 500° C. to 1300° C., maintained at1300° C. for 12 hours, then heated to 1600° C. at a rate of 400° C./hrand maintained at 1600° C. for 4 hours to carry out heat treatment. Withrespect to the properties of the obtained sintered body, the porositywas 65%, the average pore diameter was 25 μm, and the bending strengthwas 10 MPa.

Example 10 (The Present Invention)

[0066] To 70 parts of metal silicon particles having an average particlediameter of 150 μm, 30 parts of silica type inorganic hollow particleshaving an average particle diameter of 70 μm were added, and ethanol wasfurther added as a disperse medium, followed by wet mixing for 2 hoursby a ball milling and finally by drying. The obtained mixed powder waspress-molded in the same manner as in Example 1. After the molding, itwas heated in a nitrogen atmosphere (nitrogen pressure=0.08 MPa) in anelectric furnace at a rate of 60° C./hr from room temperature to 500°C., then heated to 1200° C. at a rate of 100° C./hr, maintained at 1200°C. for 24 hours, then further heated to 1400° C. at a rate of 400° C./hrand maintained at 1400° C. for 12 hours to carry out heat treatment.

[0067] The obtained sintered body had a porosity of 70% and an averagepore diameter of 60 μm, and substantial silicon metal and silicaremained in the sintered body. The thermal expansion coefficient was ahigh thermal expansion of 5.0×10⁻⁶/° C. within a range of from roomtemperature to 1000° C., and the three point bending strength was 30MPa.

Example 11 (Comparative Example)

[0068] To 90 parts of metal silicon particles having an average particlediameter of 2 μm, 5 parts of yttrium oxide having an average particlediameter of 3 μm and 5 parts of aluminum oxide having an averageparticle diameter of 1.5 μm, were added. Further, 50% by outerpercentage of deionized water and 0.1% by outer percentage, based on thepowder, of a polycarboxylic acid type dispersing agent, were added toobtain a slurry. In the slurry, a urethane resin foam of 60 mm×120 mm×30mm was immersed and deaerated under vacuum, whereupon the urethane resinfoam was taken out and dried. After drying, it was sintered in anelectric furnace at 1800° C. for 4 hours in a nitrogen atmosphere(nitrogen pressure=0.2 MPa). After the sintering, the porosity of theobtained silicon nitride filter was 75%, but the formed pores were largepores with a pore diameter of 100 μm, and the strength was a lowstrength of 8 MPa. Further, formation of silicon carbide was observedpartially in the sintered body.

Example 12 (Comparative Example)

[0069] Into toluene, 100 parts of metal silicon powder having an averageparticle diameter of 1 μm and 300 parts of polysilazane were added andthoroughly stirred to obtain a slurry. The obtained slurry was dried,and the obtained powder was pulverized, and further the particle sizewas adjusted to obtain a molding powder. In the same manner as inExample 1, it was press-molded, and then subjected to isostatic pressing(CIP) under 100 MPa. After the molding, it was heated in a nitrogenatmosphere (nitrogen pressure=0.11 MPa) at a rate of 6° C./hr from roomtemperature to 500° C., at a rate of 300° C./hr from 500° C. to 1200°C., maintained at 1200° C. for 6 hours, then further heated to 1400° C.at a rate of 100° C./hr and maintained at 1400° C. for 4 hours, to carryout heat treatment. The pore diameter of the obtained porous product wasfine at a level of 0.5 μm, and cracks of about 10 μm were present atvarious portions of the sintered body.

Example 13 (The Present Invention)

[0070] To 45 parts of metal silicon particles having an average particlediameter of 50 μm, 55 parts of alumina-silica type hollow particleshaving an average particle diameter of 100 μm, were added, and ethanolwas further added as a disperse medium, followed by wet mixing for 30minutes by a mixer and finally by drying. The obtained mixed powder waspress-molded in the same manner as in Example 1. After the molding, itwas heated in a nitrogen atmosphere (nitrogen pressure=0.09 MPa) in anelectric furnace at a rate of 400° C./hr from room temperature to 500°C., then at a rate of 60° C./hr from 500° C. to 1500° C., maintained at1500° C. for 5 hours, further heated at a rate of 300° C./hr from 1500°C. to 1700° C., and maintained at 1700° C. for 5 hours, to carry outheat treatment.

[0071] The obtained sintered body had a porosity of 87% and an averagepore diameter of 35 μm. With respect to this porous product,identification of crystal phases was carried out by X-rays, wherebypresence of silicon nitride and mullite was observed. With respect tothis porous product, the three point bending strength was measured andfound to be a low strength at a level of 5 MPa.

Example 14 (The Present Invention)

[0072] To 100 parts of metal silicon particles having an averageparticle diameter of 3 μm, 30 parts of acrylic resin particles having anaverage particle diameter of 20 μm, were added and mixed for 2 hours bya ball milling using ethyl alcohol as a disperse medium. After thedrying, this powder was filled in a press mold of 40 mm×60 mm andpress-molded under a molding pressure of 19.6 MPa to obtain a green bodyhaving a thickness of 10 mm. The green body was heated in a nitrogenatmosphere (nitrogen pressure=0.1 MPa) in an electric furnace at a rateof 60° C./hr from room temperature to 500° C., then at a rate of 400°C./hr from 500° C. to 1300° C., maintained at 1300° C. for 12 hours,then heated to 1600° C. at a rate of 400° C./hr and maintained at 1600°C. for 4 hours, to carry out heat treatment.

[0073] With respect to the properties of the obtained sintered body, theporosity was 55%, the average pore diameter was 10 μm, and the nitridingratio was 95%. Further, from the sintered body, a bending test specimenhaving a size of 4 mm×3 mm×40 mm was cut out, and the three pointbending strength with a span of 30 mm was measured at room temperature.The load-applying rate was 0.5 mm/min. As a result, the bending strengthwas 19.6 MPa.

Example 15 (The Present Invention)

[0074] The operation was the same as in Example 14 except that inExample 14, the time for maintaining at 1300° C. was changed from 12hours to 4 hours, and the time for maintaining at 1600° C. was changedfrom 4 hours to 1 hour. With respect to the properties of the obtainedsintered body, the porosity was 50%, the average pore diameter was 8 μm,and the nitriding ratio was 96%. Further, the three point bendingstrength measured in the same manner as in Example 1, was 21.6 MPa.

Example 16 (The Present Invention)

[0075] The operation was the same as in Example 14 except that inExample 14, the acrylic resin particles having an average particlediameter of 20 μm were changed to vinyl acetate resin particles havingan average particle diameter of 60 μm. With respect to the properties ofthe obtained sintered body, the porosity was 53%, the average porediameter was 20 μm, and the nitriding ratio was 95%. Further, the threepoint bending strength measured in the same manner as in Example 1 was14.7 MPa.

Example 17 (The Present Invention)

[0076] The operation was the same as in Example 14 except that inExample 14, the amount of the acrylic resin particles added, was changedfrom 30 parts to 50 parts. With respect to the properties of theobtained sintered body, the porosity was 75%, the average pore diameterwas 15 μm, and the nitriding ratio was 93%. Further, the three pointbending strength measured in the same manner as in Example 1 was 9.8MPa.

Example 18 (The Present Invention)

[0077] To 100 parts of metal silicon particles having an averageparticle diameter of 5 μm, 50 parts of acrylic resin particles having anaverage particle diameter of 100 μm, were added, and mixed for 2 hoursby a ball milling using ethyl alcohol as a disperse medium. Afterdrying, this powder was filled in a press-mold of 40 mm×60 mm andpress-molded under a molding pressure of 19.6 MPa to obtain a green bodyhaving a thickness of 10 mm. The green body was heated in a nitrogenatmosphere (nitrogen pressure=0.098 MPa) in an electric furnace at arate of 60° C./hr from room temperature to 1000° C., then at a rate of400° C./hr from 1000° C. to 1350° C., maintained at 1350° C. for 12hours, then heated to 1700° C. at a rate of 400° C./hr and maintained at1700° C. for 4 hours, to carry out heat treatment. With respect to theproperties of the obtained sintered body, the porosity was 75%, theaverage pore diameter was 19.5 μm, and the nitriding ratio was 98%.Further, the three point bending strength measured in the same manner asin Example 1, was 3.9 MPa.

Example 19 (The Present Invention)

[0078] To 100 parts of metal silicon particles having an averageparticle diameter of 1.5 μm, 40 parts of acrylic resin particles havingan average particle diameter of 50 μm, were added, and mixed for 2 hoursby a ball milling using ethyl alcohol as a disperse medium. Afterdrying, this powder was filled in a press mold of 40 mm×60 mm andpress-molded under a molding pressure of 19.6 MPa to obtain a green bodyhaving a thickness of 10 mm. The green body was heated in a nitrogenatmosphere (nitrogen pressure=0.11 MPa) in an electric furnace at a rateof 60° C./hr from room temperature to 50° C., then at a rate of 400°C./hr from 500° C. to 1200° C., maintained at 1200° C. for 12 hours,then heated to 1750° C. at a rate of 400° C./hr and maintained at 1750°C. for 2 hours, to carry out heat treatment. With respect to theproperties of the obtained sintered body, the porosity was 65%, theaverage pore diameter was 15 μm, and the nitriding ratio was 99%.Further, the three point bending strength measured in the same manner asin Example 1 was 10.8 MPa.

Example 20 (The Present Invention)

[0079] To 100 parts of metal silicon particles having an averageparticle diameter of 40 μm, 30 parts of acrylic resin particles havingan average particle diameter of 20 μm, were added, and mixed for 2 hoursby a ball milling using ethyl alcohol as a disperse medium. Afterdrying, this powder was filled in a press mold of 40 mm×60 mm andpress-molded under a molding pressure of 19.6 MPa to obtain a green bodyhaving a thickness of 10 mm. The green body was heated in a nitrogenatmosphere (nitrogen pressure=0.2 MPa) in an electric furnace at a rateof 60° C./hr from room temperature to 500° C., then at a rate of 400°C./hr from 500° C. to 1300° C., maintained at 1300° C. for 12 hours,then heated to 1600° C. at a rate of 400° C./hr and maintained at 1600°C. for 4 hours, to carry out heat treatment. With respect to theproperties of the obtained sintered body, the porosity was 65%, theaverage pore diameter was 25 μm, and the nitriding ratio was 85%.Further, the three point bending strength measured in the same manner asin Example 1 was 9.8 MPa.

Example 21 (The Present Invention)

[0080] To 100 parts of metal silicon particles having an averageparticle diameter of 100 μm, 30 parts of acrylic resin particles havingan average particle diameter of 50 μm, were added, and mixed for 2 hoursby a ball milling using ethyl alcohol as a disperse medium. Afterdrying, this powder was filled in a press mold of 40 mm×60 mm andpress-molded under a molding pressure of 19.6 MPa to obtain a green bodyhaving a thickness of 10 mm. The green body was heated in a nitrogenatmosphere (nitrogen pressure=0.1 MPa) in an electric furnace at a rateof 60° C./hr from room temperature to 500° C., then at a rate of 400°C./hr from 500° C. to 1300° C., maintained at 1300° C. for 12 hours,then heated to 1600° C. at a rate of 400° C./hr and maintained at 1600°C. for 4 hours, to carry out heat treatment. With respect to theproperties of the obtained sintered body, the porosity was 70%, theaverage pore diameter was 45 μm, and the nitriding ratio was 50%.Further, the three point bending strength measured in the same manner asin Example 1, was 4.9 MPa.

Example 22 (Comparative Example)

[0081] The operation was the same as in Example 14 except that inExample 14, no acrylic resin particles were added. With respect to theproperties of the obtained sintered body, the porosity was 20%, theaverage pore diameter was 1 μm, and the nitriding ratio was 95%.Further, the three point bending strength measured in the same manner asin Example 1, was 196 MPa.

Example 23 (The Present Invention)

[0082] A mixture obtained by blending 45 parts of metal siliconparticles having an average diameter of 50 μm, 14 parts ofalumina-silica type hollow particles having an average particle diameterof 45 μm, 9 parts of methyl cellulose as a binder, 1.5 parts of alubricant and 30.5 parts of water, was kneaded by a kneader and thenmolded into a honeycomb shape by an extrusion molding machine and dried.Then, both end surfaces were sealed in a checkered pattern so that thegas inlet side and gas outlet side would be alternating, followed bydrying again. This green body was heated in a nitrogen atmosphere(nitrogen pressure=0.15 MPa) at a rate of 60° C./hr from roomtemperature to 800° C., maintained at 800° C. for 2 hours, then heatedat a rate of 120° C./hr from 800° C. to 1350° C., and maintained at1350° C. for 8 hours, then heated at a rate of 300° C./hr from 1350° C.to 1700° C. and maintained at 1700° C. for 4 hours, to carry out heattreatment. After the heat treatment, a silicon nitride honeycomb filter(hereinafter referred to simply as a honeycomb filter) having a diameterof about 60 μm and a length of 150 mm and a cell density of 280cells/6.45 cm² at both end surfaces, was obtained.

[0083] The properties of this honeycomb filter were measured, wherebythe porosity was 56%, the average pore diameter was 10 μm, the thermalexpansion coefficient was 2.8×10⁻⁶/° C., Young's modulus was 70 GPa, andthe 1 μm pore volume ratio was 7%.

[0084] On the surface of each cell of the obtained honeycomb filter, anoxidation catalyst to burn arrested particulates, made of a noble metalelement such as platinum, other metal element or oxide, was supported toobtain a filter for particulates.

[0085] This honeycomb filter was maintained in a metal casing and thenset in an intermediate position of the exhaust gas pipe from the engine,so that the exhaust gas flowed in the honeycomb filter in a wall flowfashion in the above-mentioned filter for particulates, whereby theexhaust gas from the engine could be cleaned by the honeycomb filter.Regeneration of the honeycomb filter is carried out when the arrestedparticulates reached a predetermined amount by heating, burning andremoving the particulates by an attached heating means.

[0086] The performance of this honeycomb filter as a particulate filterwas evaluated with respect to the pressure loss change, theparticulate-arresting performance and the thermal shock resistance.

[0087] Firstly, the pressure loss change was calculated byΔP(kPa·s/cm)=ΔP₁−ΔP₀, by measuring the initial pressure loss ΔP₀ beforeuse and the pressure loss ΔP₁ after use. The initial pressure loss ΔP₀was measured by supplying nitrogen gas at a flow rate of 0.23 m³/min tothe honeycomb filter before use maintained in the metal casing. Thepressure loss ΔP₁ after use was measured in the same manner with respectto the one used for one hour after starting the engine.

[0088] The particulate arresting performance was judged by the amount ofparticulates in the exhaust gas (hereinafter referred to as cleaned gas)passed through the honeycomb filter upon expiration of 30 minutes fromthe start of the engine. Namely, the smaller the amount of particulatein the cleaned gas, the higher the particulate-arresting performance ofthe filter.

[0089] Specifically, a part of the cleaned gas was withdrawn by a gasaspirator (True-Spot Smoke Tester, trade name, manufactured by BACHARACHCompany), and the amount of particulates deposited on a white filterpaper set in the above gas aspirator, was visually evaluated in sixgrades by comparison with the attached scale. The state wherein theamount of particulates in the cleaned gas is minimum and no particulateis deposited on the filter paper (the filter paper remains to be white),is judged to be 1. On the other hand, the state wherein the particulatesare deposited over the entire surface of the filter paper, and theentire surface is black, is judged to be 6.

[0090] Further, for the thermal shock resistance, only one end of thehoneycomb filter before use was heated, and the temperature differenceΔT (° C.) between both ends when cracks were formed in the honeycombfilter, was measured.

[0091] The evaluation results in a case where the above filter was usedas a filter for an exhaust gas-cleaning apparatus, are shown in Table 1.

[0092] In the Table, filter 1 is the above filter, and filter 2 andfilter 3 are honeycomb filters made of silicon carbide having the sameshape. With respect to the properties of filter 2, the porosity was 38%,the average pore diameter was 31 μm, the thermal expansion coefficientwas 4.2×10⁻⁶/° C., the Young's modulus was 220 GPa, and the 1 μm porevolume ratio was 0.1%, and with respect to the properties of filter 3,the porosity was 48%, the average pore diameter was 9 μm, the thermalexpansion coefficient was 4.2×10⁻⁶/° C., the Young's modulus was 250GPa, and the 1 μm pore volume ratio was 0.5%. TABLE 1 ParticulateThermal shock Pressure loss arresting resistance ΔT change ΔPperformance Filter 1 500 0.2 1 Filter 2 300 0.3 3 Filter 3 350 0.7 1

Industrial Applicability

[0093] By the method for producing a silicon nitride filter of thepresent invention, it is possible to produce a silicon nitride filterwhich has properties excellent in e.g. heat resistance, thermal shockresistance, corrosion resistance, chemical resistance and mechanicalstrength and which is suitable as a dust arresting and dust removingfilter at a high temperature or in a corrosive atmosphere.

[0094] Further, the silicon nitride filter of the present invention notonly has heat resistance, corrosion resistance, and acid resistance, butalso has a low Young's modulus and thus is excellent in thermal shockresistance. Further, it has an average pore diameter suitable forarresting particulates, etc., and also has a high porosity andmechanical strength, whereby it is most suitable as a filter forparticulates.

[0095] The entire disclosures of Japanese Patent Application No.11-366305 filed on Dec. 24, 1999, and Japanese Patent Application No.11-366306 filed on Dec. 24, 1999 including specifications, claims andsummaries are incorporated herein by reference in their entireties.

1. A method for producing a silicon nitride filter, characterized byheat-treating in nitrogen a green body comprising from 40 to 90 mass %of metal silicon particles having an average particle diameter of from 1to 200 μm and from 10 to 60 mass % of a pore-forming agent, providedthat the total amount of the metal silicon particles and thepore-forming agent is at least 90 mass %, to form a porous product madesubstantially of silicon nitride.
 2. The method for producing a siliconnitride filter according to claim 1, wherein the heat-treatingconditions are such that heat treatment of a first step is carried outby maintaining the green body in a nitrogen atmosphere at a temperatureof from 1,000 to 1,400° C. for from 4 to 24 hours, and then, heattreatment of a second step is further carried out by maintaining it at atemperature of from 1,450 to 1,800° C. for from 1 to 12 hours.
 3. Themethod for producing a silicon nitride filter according to claim 1,wherein the green body comprises from 40 to 85 mass % of metal siliconparticles having an average particle diameter of from 5 to 200 μm andfrom 15 to 50 mass % of a pore-forming agent, and the pore-forming agentis oxide ceramic hollow particles.
 4. The method for producing a siliconnitride filter according to claim 3, wherein the oxide ceramic hollowparticles contain, as the main component, an oxide of at least one metalselected from the group consisting of Al, Si, Ca, Sr, Ba and Mg.
 5. Themethod for producing a silicon nitride filter according to claim 3,wherein the average pore diameter as measured by a mercury immersionmethod of the porous product is from 5 to 40 μm.
 6. The method forproducing a silicon nitride filter according to claim 3, wherein theheat-treating conditions are such that heat treatment of a first step iscarried out by maintaining the green body in a nitrogen atmosphere at atemperature of from 1,200 to 1,400° C. for from 4 to 12 hours, and then,heat treatment of a second step is further carried out by maintaining itat a temperature of from 1,500 to 1,800° C. for from 1 to 12 hours. 7.The method for producing a silicon nitride filter according to claim 1,wherein the green body comprises from 40 to 90 mass % of metal siliconparticles having an average particle diameter of from 1 to 30 μm andfrom 10 to 50 mass % of a pore-forming agent, and the pore-forming agentis a flying-type pore-forming agent.
 8. The method for producing asilicon nitride filter according to claim 7, wherein the amount ofsilicon contained as silicon nitride based on the total silicon in thesilicon nitride filter, is at least 90%.
 9. The method for producing asilicon nitride filter according to claim 1, wherein the porosity of theporous product is from 30 to 80%.
 10. The method for producing a siliconnitride filter according to claim 3, wherein the porosity of the porousproduct is from 30 to 80%.
 11. The method for producing a siliconnitride filter according to claim 7, wherein the porosity of the porousproduct is from 30 to 80%.
 12. The method for producing a siliconnitride filter according to claim 7, wherein the average pore diameteras measured by a mercury immersion method of the porous body is from 5to 20 μm.
 13. The method for producing a silicon nitride filteraccording to claim 7, wherein the heat-treating conditions are such thatheat treatment of a first step is carried out by maintaining the greenbody in a nitrogen atmosphere at a temperature of from 1,000 to 1,400°C. for from 5 to 24 hours, and then, heat treatment of a second step isfurther carried out by maintaining it at a temperature of from 1,450 to1,800° C. for from 2 to 5 hours.
 14. A silicon nitride filtercharacterized in that the porosity is from 40 to 70%, and the cumulativepore volume of pores with diameters of at most 1 μm is from 1 to 15 vol% of the total pore volume.
 15. The silicon nitride filter according toclaim 14, which has a Young's modulus of from 20 to 100 GPa and athermal expansion coefficient of at most 4×10⁻⁶/° C.